How many three digit numbers can be made up of 1, 2, 3, 4, 5 and 6, which have no repetition and whose individual digit is 5?

How many three digit numbers can be made up of 1, 2, 3, 4, 5 and 6, which have no repetition and whose individual digit is 5?

Let the single digit be 5 and must be three digits, that is to say, use 1, 2, 3, 4 to form different two digits. 12, 13, 14, 16; 21, 23, 24, 26; 31, 32, 34, 36; 41, 42, 43, 46; 61, 62, 63, 64; a total of 4 × 5 = 20 (species) answer: use 1, 2, 3, 4, 5, 6 to form 20 three digits without repeating numbers and the single digit is 5